Transparent conducting films (TCFs) exhibit high electrical conductance and large optical transparency in a specified spectral range, such as ultraviolet, visible and infrared region, and are widely used in flat-panel display, touch panel displays, photovoltaic devices, light emitting diodes (LEDs), smart window technology, photodetectors and other optoelectronic devices.
The first widespread use of TCFs was as transparent electrical heaters for aircraft windshield de-icing in World War II by a partially oxidized cadmium film. Later, metal thin films could no longer meet the increasing high requirement of TCFs application. Besides the factor that metal thin films quickly tarnish in air and moisture, the electron mean free path (EMFP) of metal is usually much larger than the maximum thickness allowed for high transparency. In 1938, Fuchs showed that the resistivity of a thin film was significantly higher than that of the corresponding bulk material due to strong surface scattering, when the EMFP became smaller than the thickness of the film. See Fuchs, K. in Proc. Cambridge Philos. Soc. 100 (Cambridge Univ. Press). For example, the EMFP of silver (ρbulk=1.59 μΩ cm) was 52 nm at room temperature.
To date degenerately doped wide band gap semiconductors, such as Al doped ZnO (AZO), tin doped indium oxide (ITO) and SnO2 are widely used as TCF. ITO is the most widely used TCFs on the market. For example, the ITO-based thin Si solar cell has much higher fill factor (FF) than that of AZO-based one due to its high conductance A high FF increases the solar cell efficiency. The maximum power Pmax generated by a solar cell is given by Pmax=Voc Isc FF, with Voc the open circuit voltage and Isc the short circuit current. Higher fill factors leads to an increase in maximum power generated.
TCFs are optimized materials where doping close to the solubility limit is performed to maximize carrier concentration, while a high carrier mobility is ensured by choosing materials where the cation is a post transition metal, such as Zn2+, Cd2+, In3+or Sn4+ (referred to as transparent conducting oxide (TCO) cations) and the conduction band is derived from the hybridization of the strongly delocalized s orbital and the oxygen p orbitals. Conduction bands have mainly s-orbital character, exhibiting a large band width and thus a small effective mass (typically below 0.3 free electron masses). These TCO materials allow for higher carrier concentration in excess of ˜mid 1020 cm−3 with high room temperature mobilities typically around 50˜60 cm2/V-s, which are highly desirable for the application of transparent conductors and currently strike the best balance of highest electrical conductivities and plasma frequencies below the visible spectrum (wP<˜1 eV). See R. G. Gordon, MRS Bulletin 25, 52 (2000).
However, the supply of ITO to meet the rapidly increased demand for ITO has become a critical issue recently. The low abundance of In, has driven the price up, motivating people to search for alternative TCF materials. Higher conductivity is also needed for highly scaled structures, expanding the transmission window both into the UV (in half tone layers of phase shift masks for photolithography and future biotechnology such as transparent electrode for electrical sensing analysis of deoxyribonucleic acid (DNA) on a lab-on-a-chip, water sterilization) and near IR for (sensing, optical communication) are currently needed to enable other applications. However, further increasing the carrier concentration is limited by the solubility of the dopant atoms into the semiconducting host material, and increasing dopant concentration beyond ˜mid 1020 cm−3 can result in compensating doping as well as considerably reduced carrier mobility at room temperature due to enhanced impurity scattering. Conductivities larger than 104 S/cm are barely exceeded for ITO.
Both, scaling film thickness and expanding the transmission window into the near IR and UV can be achieved by reducing the sheet resistance Rs. The figure of merit of transparent electrode is defined by sigma/alpha=−{Rs In(T±R)}−1. See R. G. Gordon, MRS Bulletin 25, 52 (2000). To develop new TCF materials with a good FOM new strategies are required that are beyond the two design schemes of scaling conventional metals (Ag, Au, Al, Cu) and degenerately doped wide band gap semiconductors (ITO, AZO, SnO2).
In this regard, Carbon based nanostructured materials have been proposed, such as graphene grown by chemical vapor deposition, and carbon nanotube meshes as well as metal nanotrough network and combinations thereof (metal mesh+ultrathin ITO). See Bae, S. et al., Nature nanotechnology 2010, 5, (8), 574-578; Wu, Z. et al., Science 2004, 305, (5688), 1273-1276.; Wu, H. et al., Nature nanotechnology 2013. However, due to the high synthesis and processing incompatibilities with existing and established fabrication routes and thus additional high costs, conventional thin film materials, namely degenerately doped wide band semiconductors and ultrathin metal thin films, are still the main workhorse as transparent electrode in industry. These new processes and materials have not been applied at large scale and the economics, supply and demand might negatively impact the feasibility of these lab-size tested approaches when transitioning to large quantities.
Ultimately, a new transparent conducting material has to be made out of abundant elements, should be chemically inert and have low toxicity, that is easy to integrate and straight forward to process, and should be fully compatible with current large volume TCFs production methods to enable utilization of existing equipment and ensure minimal capital investment by switching the existing PVD process capabilities to the new material. TCFs with higher conductivity and equal or better optical transmission in the wavelength interval of interest would allow manifold increase of throughput by reducing the film thickness required to maintain the same sheet resistance.
A need still exists for materials exhibiting a much higher electrical conductivity than degenerately doped semiconductor (>5×103 S/cm), which should rather be on the order of ˜5×104 S/cm, therefore closing the gap to metals, while having a plasma frequency that is much lower than that of metals (˜>5 eV) and rather comparable to degenerately doped semiconductors (˜1 eV). Both requirements are competing demands, since both quantities depend on the ratio of carrier concentration and carrier effective mass n/m*. In fact, a large materials design space for tuning the plasma frequency and electrical conductivity through both, effective mass and free carrier concentration is highly desirable to increase the optical transparency in the IR range, while maintaining a high electrical conductivity. Expanding the transparent window into the UV range requires the suppression of optical interband transition, which can be achieved by ensuring the same occupancy of initial and final electronic state involved in the absorption of light at a specific wavelength.
US 2015/0123046 discloses transparent conductive thin film and an electronic device including the same. The transparent conductive thin film may include a perovskite vanadium oxide represented by A1-x VO3±δ, where A is a Group II element, 0≤x<1, and δ is a number necessary for charge balance in the oxide.
However, a need still exists for transparent conductive thin film materials having improved optical and electrical properties to make them even thinner, more conductive and yet more optically transparent.